In a typical radio communications system, user communications terminals referred to as User Equipment's (UEs) communicate via. Radio Access Network (RAN) with other networks like Internet. The RAN covers a geographical area which is divided into cell areas, with each cell area being served by a Base Station (BS), e.g., a radio BS (RBS), which in some networks is also called a “NodeB” or enhanced Node B (i.e., eNodeB or eNB).
In conventional methods and systems, MIMO techniques are used to increase capacity of a wireless system without increasing bandwidth and power consumption. This is achieved by multiplexing the data of multiple users over spatial channels, when multiple antennas are made available at a transmitter and a receiver. The number of independent data streams that can be multiplexed is limited by a minimum number of antennas at the transmitter and receiver. Thus, multiplexing gain that can be achieved in a single user MIMO system is limited by the number of antennas at the receiver due to its form factor. In order to overcome the limitation as described above, the conventional methods and systems considers the multiple antennas available at multiple UEs as a larger MIMO system. The signals to be transmitted should be precoded preferably with orthogonal precoders, or otherwise, there can be a leakage from co-scheduled user signals at the receiver. As there are multiple users in the system, the limitation can be on the number of antennas available at the transmitter.
Due to the advancement of smart antenna technologies, an active antenna with large number of antenna elements can be arranged in two dimensional arrays at the transmitter. Further, the entire or a subset of the antenna elements can be configured to form a directional beam and can create multiple directional beams. Generally, placing large number of antennas at the transmitter is a common setup in Massive or Full Dimensional MIMO (FD-MIMO) systems. Therefore, it is possible to exploit the capacity of such large multi-user MIMO (MU-MIMO) system using appropriate transmission technique, and other related supporting features like precoders, reference signals, signaling, Channel Quality Indicator (CQI), measurement, feedback, link adaptation, and receivers.
In general, closed-loop MU-MIMO system requires Channel State Information (CSI) at the transmitter (CSIT) to form a beam directed to the receiver. The same precoding information plays a crucial role in pairing the receivers during scheduling for the closed-loop multi-user system. Achieving full CSIT in a Frequency Division Duplex (FDD) system is nearly impractical, and also in a Time Division Duplex (TDD) system, where the channel reciprocity holds, the challenge is different like channel estimation from the contaminated signals at the reference signal locations due to interference. Even achieving the partial CSIT with large number of active users will lead to affect the uplink throughput because of the overhead in a feedback channel In addition, the closed loop operation demands high coherence time (i.e., low mobility) in order to suite the channel conditions with the reported CSI.
In the conventional methods and systems, when the receiver is not moving or moving at a slow speed, the receiver estimates the channel and the precoder; and reports the estimated precoder to the transmitter. After receiving the precoder, the transmitter utilizes the precoder for its data transmission. Consider a scenario where the receiver is moving at a higher speed. If the receiver estimates and reports the precoder to the transmitter then by the time the transmitter receives the precoder and applies it, the channel of the receiver changes thus making the reported precoder meaningless.
The above information is presented as background information only to help the reader to understand the present invention. Applicants have made no determination and make no assertion as to whether any of the above might be applicable as Prior Art with regard to the present application.